You never want to have a persona that is weak to fire equipped while fighting her.

Here's a pro-strat, get a slime and level it until it learns Red Wall, and then fuse it into something else that is neutral or resist fire such as Orobas. Try to keep Red Wall up on Chie the whole time to keep her from getting knocked down and Yukiko from getting extra turns---It's over, you've lost the game. -NearI don't remember Redd White changing his name to Bandit Keith. Not even in America.-Aceburner

Ok I thought it was the characters themselves that were weak against those types not their personas.

I'm dumb.

Their Personas define what they are weak to. Every character is bound to their Persona, EXCEPT the main character. So Chie is always going to be weak to fire, and Yosuke will always be weak to electricity.

To solve the issue with Chie, make a Persona who knows Red Wall, to give her a temporary resistance to Fire.

When you face a boss and find out what element their main attacks are- don't use a Persona on the MC that is weak to that attack. If you one thats strong to that element (or even absorbs its attack type) all the better.

Other party members matter less- but always make sure your MC is never weak to whatever you are facing (which you can do since you have the luxury of switching his persona, and therefore his stats).